An organic EL device can be classified into two types, i.e. a fluorescent EL device and a phosphorescent EL device according to its emission principle. When a voltage is applied to an organic EL device, holes are injected from an anode, and electrons are injected from a cathode, and holes and electrons recombine in an emitting layer to form excitons. As for the resulting excitons, according to the electron spin statistics theory, they become singlet excitons and triplet excitons in an amount ratio of 25%:75%. Therefore, in a fluorescent EL device which uses emission caused by singlet excitons, the limited value of the internal quantum efficiency is believed to be 25%. A technology for prolonging the lifetime of a fluorescent EL device utilizing a fluorescent material has been recently improved. This technology is being applied to a full-color display of portable phones, TVs, or the like. However, a fluorescent EL device is required to be improved in efficiency.
In association with the technology of improving the efficiency of a fluorescent EL device, several technologies are disclosed in which emission is obtained from triplet excitons, which have heretofore been not utilized effectively. For example, in Non-Patent Document 1, a non-doped device in which an anthracene-based compound is used as a host material is analyzed. A mechanism is found that singlet excitons are formed by collision and fusion of two triplet excitons, whereby fluorescent emission is increased. However, Non-Patent Document 1 discloses only that fluorescent emission is increased by collision and fusion of triplet excitons in a non-doped device in which only a host material is used. In this technology, an increase in efficiency by triplet excitons is as low as 3 to 6%.
Non-Patent Document 2 reports that a blue fluorescent device exhibits an internal quantum efficiency of 28.5%, exceeding 25%, which is the conventional theoretical limit value. However, no technical means for attaining an efficiency exceeding 25% is disclosed. In respect of putting a full-color organic EL TV into practical use, a further increase in efficiency has been required.
In Patent Document 1, another example is disclosed in which triplet excitons are used in a fluorescent device. In normal organic molecules, the lowest excited triplet state (T1) is lower than the lowest excited singlet state (S1). However, in some organic molecules, the triplet excited state (T2) is higher than S1. In such a case, it is believed that emission from the singlet excited state can be obtained due to the occurrence of transition from T2 to S1. However, actually, the external quantum efficiency is about 6% (the internal quantum efficiency is 24% when the outcoupling efficiency is taken as 25%), which does not exceed the value of 25% which has conventionally been believed to be the limit value. The mechanism disclosed in this document is that emission is obtained due to the intersystem crossing from the triplet excited state to the singlet excited state in a single molecule. Generation of single excitons by collision of two triplet excitons as disclosed in Non-Patent Document 1 is not occurred in this mechanism.
Patent Documents 2 and 3 each disclose a technology in which a phenanthroline derivative such as BCP (bathocuproin) and BPhen is used in a hole-blocking layer in a fluorescent device to increase the density of holes at the interface between a hole-blocking layer and an emitting layer, enabling recombination to occur efficiently. However, a phenanthroline derivative such as BCP (bathocuproin) and BPhen is vulnerable to holes and poor in resistance to oxidation, and the performance thereof is insufficient in respect of prolonging the lifetime of a device.
In Patent Documents 4 and 5, a fluorescent device is disclosed in which an aromatic compound such as an anthracene derivative is used as a material for an electron-transporting layer which is in contact with an emitting layer. However, this is a device which is designed based on the mechanism that generated singlet excitons emit fluorescence within a short period of time. Therefore, no consideration is made on the relationship with the triplet energy of an electron-transporting layer which is normally designed in a phosphorescent device. Actually, since the triplet energy of an electron-transporting layer is smaller than the triplet energy of an emitting layer, triplet excitons generated in an emitting layer are diffused to an electron-transporting layer, and then, thermally deactivated. Therefore, it is difficult to exceed the theoretical limit value of 25% of the conventional fluorescent device. Furthermore, since the affinity of an electron-transporting layer is too large, electrons are not injected satisfactorily to an emitting layer of which the affinity is small, and hence, improvement in efficiency cannot necessarily be attained. In addition, Patent Document 6 discloses a device in which a blue-emitting fluoranthene-based dopant which has a long life and a high efficiency. This device is not necessarily highly efficient.
Meanwhile, a phosphorescent device directly utilizes emission from triplet excitons. Since the singlet exciton energy is converted to triplet excitons by the spin conversion within an emitting molecule, it is expected that an internal quantum efficiency of nearly 100% can be attained, in principle. For the above-mentioned reason, since a phosphorescent device using an Ir complex was reported by Forrest et al. in 2000, a phosphorescent device has attracted attention as a technology of improving efficiency of an organic EL device. Although a red phosphorescent device has reached the level of practical use, green and blue phosphorescent devices have a lifetime shorter than that of a fluorescent device. In particular, as for a blue phosphorescent device, there still remains a problem that not only lifetime is short but also color purity or luminous efficiency is insufficient. For these reasons, phosphorescent devices have not yet been put into practical use.
As a method for obtaining a full-color organic EL device, an emitting layer is patterned to provide a blue-emitting fluorescent layer, a green-emitting phosphorescent layer and a red-emitting phosphorescent layer. If peripheral layers other than an emitting layer are used as the common layer for the three emitting layers, the production steps are reduced, thereby to facilitate mass production. However, the blue-emitting fluorescent layer, the green-emitting phosphorescent layer and the red-emitting phosphorescent layer largely differ in physical value of constituent materials, for example, affinity, ionization potential, energy gap or the like. When peripheral layers are used as the common layer, a configuration is made in which optimum carrier injection performance can be attained in the green-emitting phosphorescent layer of which the energy gap is the largest. Therefore, other emitting layers (in particular, blue-emitting fluorescent layer) have deteriorated performance.
Patent Document 9 discloses a device comprising a blue emitting portion containing a fluorescent dopant, a green emitting portion containing a phosphorescent dopant and a red emitting portion containing a phosphorescent dopant, in which a hole-blocking layer is provided as the common layer.
In this device, by using a hole-blocking layer as the common layer, the production steps are reduced. However, use of a hole-blocking layer as the common layer, electron injection from the hole-blocking layer to each emitting layer has become a problem to be solved. Actually, difference in affinity level between a blue emitting portion and a hole-blocking layer is as small as about 0.2 eV. Since a material having a small affinity such as CBP is used in a green emitting portion, difference in affinity between the green emitting portion and the hole-blocking layer is as large as about 0.6 eV. Therefore, electron-injection properties are lowered in the green emitting portion, whereby a driving voltage is increased. Furthermore, since a recombination region is concentrated in the interface between a green phosphorescent emitting layer and a hole-blocking layer, excitons are significantly diffused, thereby inhibiting improvement in luminous efficiency of a green emitting portion.
Patent Document 10 discloses an organic EL device in which difference in affinity ΔAf between an emitting layer containing a phosphorescent-emitting dopant and an electron-transporting layer satisfies the relationship 0.2<ΔAf≦0.65 eV. However, in this technology, no disclosure is made on improvement in efficiency of the emitting layer when patterning of a blue emitting portion, a green emitting portion and a red emitting portion is performed.